Pope Benedict XVI has left the Vatican as head of the Catholic Church for the last time, becoming only the second pontiff to resign because he felt unable to fulfil his duties in the Church's 2,000-year history.

Pope Benedict left the Vatican in a helicopter and travelled to Castel Gandolfo, where he will spend the next few months.

The German Pope stunned the globe when he announced on February 11 his surprise decision to step down, saying he no longer had the "strength of mind and body" to carry on in a fast-changing world.

A shy theologian who struggled with Vatican infighting and a raft of toxic sexual abuse scandals, Benedict said his eight-year pontificate had seen "sunny days" and "stormy waters", but he added: "I never felt alone".

The only other pope who resigned by choice was Celestine V, a humble hermit who stepped down in 1294 after just a few months in office out of disgust with Vatican corruption and intrigue.

Once Benedict takes up permanent residence in a former convent on a hill within the Vatican walls, the Church will find itself in the unprecedented situation of having a pope and his predecessor living within a stone's throw of each other.

Vatican analysts have suggested his sudden exit could set a precedent for ageing popes in the future, and many ordinary Catholics say a more youthful, pastoral figure could breathe new life into a Church struggling on many levels.

From Catholic reformers calling for women clergy and for an end to priestly celibacy, to growing secularism in the West and ongoing scandals uncovering sexual abuse by paedophile priests going back decades, the next pope will have a tough agenda.

French cardinal Jean-Pierre Ricard, one of the electors, spoke of the upcoming conclave in an interview with Italian daily Il Messaggero saying: "Our eyes will be turned on the conditions of the world, to the great challenges the Church faces."

According to Church rules, any Catholic adult male can be elected pope - but the last non-cardinal to land the top job was Urban VI in the late 14th century.